The present relates to apparatus for measuring the density of materials and, more particularly, relates to an apparatus and method for automating the calibration process of such density-measuring apparatus.
Nuclear radiation gauges have been widely used for measuring the density of soil and asphaltic materials. Such gauges typically include a source of gamma radiation which directs gamma radiation into the test material, and a radiation detector located adjacent to the surface of the test material for detecting radiation scattered back to the surface. From this detector reading, a determination of the density of the material can be made. Examples of such gauges are described in U.S. Pat. No. 2,781,453 and U.S. Pat. No. 3,544,793, both of which are incorporated by reference herein in their entirety.
Nuclear density gauges currently in use, for example, the Troxler Model 3400 and 4400 series gauges manufactured by the assignee of the present invention, employ a nuclear radiation source, typically a mono-energetic source, that discharges gamma radiation into the test specimen and a radiation detector, typically a Geiger Mueller tube, that measures the scattered radiation. The gamma radiation interacts with matter in the test specimen, either by losing energy and changing direction (Compton interactions) or by terminating (photoelectric interactions). Consequently, the gamma radiation detected by the radiation detector has a continuous energy spectrum.
These gauges are designed to operate both in a xe2x80x9cbackscatterxe2x80x9d mode and in a direct transmission mode. The radiation source is vertically moveable from a backscatter position where it resides within the gauge housing to a series of direct transmission positions where it is inserted into small holes or bores in the test specimen. The gamma radiation received by the radiation detector is related to the density of the test medium by an expression of the following form.
CR=Aexp(xe2x88x92BD)xe2x88x92Cxe2x80x83xe2x80x83Equation 1
where:
CR=count ratio (the accumulated photon count normalized to a reference standard photon count for purposes of eliminating long term effects of source decay and electronic drift),
D=density of test specimen, and
A, B, and C are constants.
The gauges are factory calibrated to arrive at values for constants A, B, and C for each gauge at each source depth position. The factory calibration procedure is a time-consuming iterative process, which may require several hours, or even days, to complete. In order to determine values for the three calibration parameters of the above equation, count measurements must be taken using at least three materials of different densities at each radiation source position. Typically, the three materials are solid blocks of aluminum, magnesium and a laminate of magnesium and aluminum. In some instances, as many as five calibration blocks of material have been employed in order to take into account the distinct mass attenuation coefficients of different soils. Thus, the standard factory calibration methods, often referred to as the three-block or five-block calibration methods, require a large number of individual counts in order to complete the calibration. For example, a gauge having a twelve-inch radiation source rod with seven different radiation source depth positions requires a minimum of twenty-one separate counts using the three-block calibration method. Each count is taken for a predetermined period of time, with longer periods of time producing greater precision. For example, for some gauge models, a typical count period for calibration is about four minutes for a direct transmission mode and about eight to twenty minutes for backscatter mode. Once all the counts are accumulated, values for the calibration parameters A, B, and C are calculated for each radiation source position.
The above-described calibration method is both time consuming and labor intensive because it requires numerous counts and movement of the gauge to positions overlying a plurality of blocks. To better automate the process and remove the need for numerous blocks, an automated calibration apparatus and method has been developed and described in PCT Publication No. WO 00/45159, assigned to the assignee of the present invention. The PCT application, which is incorporated herein by reference in its entirety, describes a calibration apparatus capable of simulating a plurality of densities at each radiation source depth, eliminating the need for movement of the gauge from block to block. However, there remains a need in the art for a method of further automating the calibration process.
The present invention facilitates automation of the calibration process for a nuclear gauge by allowing the source rod to be moved from one predetermined source rod position to the next without manual repositioning of the source rod by the user. When used in conjunction with an automated calibration apparatus, the present invention enables full automation of the calibration process.
The apparatus is advantageously used with a nuclear gauge, wherein the gauge comprises a gauge housing, a longitudinally moveable source rod extending into said gauge housing and including a handle affixed to a distal end thereof, the handle having a cavity therethrough and including an indexer, and an index rod extending through the cavity in the handle and affixed within the gauge housing. The index rod includes a plurality of notches positioned for engagement with the indexer of the handle, each notch corresponding to a predetermined source rod position. A source rod grip can be temporarily affixed to the source rod and operatively connected to a motorized linear actuator such that linear motion may be imparted to the source rod grip. In a preferred embodiment, the linear actuator is affixed to the index rod of the gauge.
One embodiment of the apparatus of the invention comprises a linearly moveable member, such as a threaded rod, and a motorized linear actuator, such as a stepper motor, operatively connected to the linearly moveable member for imparting linear motion to the member. The invention further includes a source rod grip attached to the linearly moveable member for affixing the source rod to the member. Use of a linearly moveable member is a convenient method of connecting the linear actuator to the source rod grip so that linear motion may be imparted to the source rod grip and, consequently, to the source rod itself. As would be understood, the source rod grip can be configured for affixation directly to the source rod or to any part affixed to the source rod, such as a handle.
The apparatus may further comprise a tube operatively positioned to house a distal end of the linearly moveable member. The tube has a linear notch extending in the direction of travel of the linearly moveable member. A pin is affixed to the distal end of the linearly moveable member. The pin extends through the notch, thereby preventing axial rotation of the distal end of the linearly moveable member. This is a particularly useful embodiment wherein the linearly moveable member is a threaded rod.
A preferred embodiment of the source rod grip comprises a first vice jaw and a second vice jaw. The two vice jaws are attached and slidably engaged so that the vice jaws may be spaced apart. There is at least one pin affixed to each vice jaw and operatively positioned to grip the handle of a source rod of a nuclear gauge. The source rod grip further includes a tightening screw threaded into the vice jaws for adjusting the spacing therebetween.
A mounting plate can be affixed to the motorized linear actuator, the mounting plate having at least one hole therethrough. The apparatus can further include an enclosure surrounding the linear actuator, the enclosure including a bottom plate having one or more posts affixed thereto. The posts have a flanged end distal from the bottom plate that pass through the holes in the mounting plate. Thus, the mounting plate is moveable between the bottom plate and the flanged end of the post. Springs are mounted around the posts between the mounting plate and the flanged end of the post such that the mounting plate is biased towards the bottom plate. A switch is positioned to engage the mounting plate when the spring is substantially compressed against the flanged end of the post.
A processor is preferably connected to the motorized linear actuator. The processor may include an instruction set. A preferred instruction set includes a first instruction to raise the source rod grip until the indexer of the handle rises from a notch corresponding to a first predetermined source rod position to a position above a notch corresponding to a second predetermined source rod position, and a second instruction to lower the source rod grip until the indexer of the handle engages the notch corresponding to the second predetermined source rod position.
In operation, the invention enables a source rod of a nuclear gauge to be moved into a plurality of source rod positions. As noted above, the method and apparatus of the invention are particularly well-suited for use with a nuclear gauge comprising a longitudinally moveable source rod and an index rod adjacent thereto. In the method of the invention, the source rod is temporarily affixed into a first predetermined source rod position and a source rod grip is attached to the source rod. The source rod grip is moved using a linear actuator such that the source rod is temporarily affixed in a second predetermined source rod position. This moving step can be repeated in order to temporarily affix the source rod in each remaining source rod position.
If the gauge includes a handle affixed to the source rod, wherein the handle includes an indexer operatively positioned for engaging the notches of the index rod in order to temporarily affix the source rod in one of the predetermined positions, the moving step may include raising the source rod grip using the linear actuator such that the indexer is positioned above the notch in the index rod corresponding to a second predetermined source rod position. The source rod grip is then lowered using the linear actuator until the indexer is engaged with the notch in the index rod corresponding to the next determined source rod position. If the linear actuator is mounted on a moveable plate, the method may further include continuing to attempt to lower the source rod grip after the indexer is engaged with the notch, thereby raising the moveable plate from an initial position to a second position. The linear actuator stops when the moveable plate contacts a switch position to engage the moveable plate at the second position.